Visual prominence meter

ABSTRACT

Method for a visual measurement, in which an image sector of an image with a target object is presented to an eye of an operative by a display unit, and in which the fovea of the eye is placed into such a reference position relative to the display unit, or the image sector is shifted in such a way towards the periphery relative to the fovea of the eye in a reference position, that the target object lies outside the fovea and the operative decides whether the target object is still visible, and the extent to which the target object is situated outside the reference position is determined.

BACKGROUND OF THE INVENTION

The invention relates to both a visual measuring method and a visualmeasuring device, in particular for determining the visual prominence ofa target object within an image. The invention can be carried out bothwith purely optical techniques and with all kinds of combinations ofoptoelectronic techniques. The invention is explained below inparticular, but not restrictively, with reference to exemplaryembodiments on the basis of purely optical techniques.

SUMMARY OF THE INVENTION

There has long been a desire to determine as objectively as possible thevisual prominence of a target object, for example a traffic sign, withinan image, for example a street scene. The object of the invention is toprovide both a method and a device by means of which that desire can befulfilled.

On the one hand, the invention is based on the realization that theobjective measure used for visual prominence can be the extent to whicha target object situated in an image is moved towards the periphery ofthe field of vision of an eye while the operative is still just capableof perceiving the target object in the periphery of his field of vision.On the other hand, the invention is based on the idea that thereliability of the objective measurements increases when the image, withthe target object therein in each case, is presented for a relativelyshort time to the operative. It is expected that disturbing subjectivevalue judgments will be filtered out as well as possible in this way.The invention is also based on the idea that the extent of a contrastreduction in the image sector presented to the eye can be a secondarymeasure of determining the visual prominence of a target object withinan image.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is explained in greater detail below on the basis ofnon-restrictive exemplary embodiments with reference to the drawings, inwhich:

FIG. 1 shows diagrammatically an optical measuring device according to afirst embodiment of the invention, in which only the main parts areshown;

FIG. 2 shows a diagrammatic view of a second variant of an embodiment ofan optical measuring device according to the invention, illustrating atarget object, and in which only the main parts are shown; and

FIG. 3 shows a diagrammatic view of a third variant of an embodiment ofthe measuring device according to the present invention, including anillustration of its housing, and showing only the main parts.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The measuring device 1 shown in FIG. 1 comprises essentially two mirrors2, 3 and three lenses 4, 5 and 6. A diaphragm 7 with an adjustablediaphragm aperture is also present. The light path from the image (notshown) to the eye 9 is indicated diagrammatically by thedashed-and-dotted line 8. An image can be viewed with the eye 9 in afixed position, for example assisted in this by an eye fixation pointwhich is accommodated in, for example, the image plane of the lens 6. Bymeans of the tilting mirror 3, the image can be viewed at two differentangles, so that the target object is projected at two different pointsonto the retina of the eye 9. With the diaphragm 7 it is also possibleto present the whole image to the eye 9 (diaphragm 7 opened fully), orto present just a detail of that image (diaphragm 7 opened onlyslightly). It is now ensured that with the eye 9 in the referenceposition the detail in the image, with essentially only the targetobject, is projected onto or essentially onto the fovea of the retina,so that the detail of the image is situated in a "center forward"position in front of the eye 9. If the mirror 3 is now tilted through apredetermined angle and the diaphragm 7 is opened fully at the sametime, the full image is presented to the eye 9, but with the targetobject shifted towards the periphery of the field of vision of the eye9. The operative now decides whether in that situation, with his eye 9still directed at the reference point, he can or cannot perceive thetarget object in the periphery of his field of vision. The angle throughwhich the mirror 3 is tilted is adjusted, following which the imagedetail and the full image are again presented alternately in succession.The angle through which the mirror 3 is tilted so that the operative canperceive or just fails to be able to perceive the target object in theperiphery of his field of vision can be used as a measure of the visualprominence of that target object within the image in question. By way ofexample, a variant is also shown in FIG. 1, where a diaphragm 10 shownin an imaginary way replaces the diaphragm 7. Other variants are alsoconceivable, for example in which the mirror 2 can be tilted. Instead ofmaking the mirror 3 tilt relatively quickly between its two endpositions, in which case the aperture in the diaphragm 7 is preferablyadjusted simultaneously, the following variant is also conceivable,based on a slightly adapted measuring method: any diaphragm is nowdispensed with, or is placed in such a position that a large part of theimage, with the target object therein as one of the parts, is presentedto the eye 9. The optical system is placed in such a position that thetarget object is situated initially in the center of the image. Thetarget object is now gradually shifted towards the periphery of thefield of perception by striving for a gradual adjustment in the opticalsystem, for example by gradually tilting the mirror 3. The angulardisplacement of the mirror 3 at the moment when the operative, with hiseye 9 directed at the reference point in the optical system, still justperceives or just fails to perceive the target object in the peripheryof his field of vision is a measure of the visual prominence of thattarget object within that image.

When two image sectors are presented alternately in succession, it ispreferable first to present an image sector in which the target objectis projected onto the fovea of the eye directed at a fixation point, andthen, by depressing, for example, an exposure lever to present thesecond image sector with the target object shifted towards the peripheryimmediately after the first image sector, and for a relatively shorttime in the range from 1/10 to 10 s, preferably 1/8 to 8 s.

FIG. 2 shows a further variant of the invention by means of which twoimages can be presented in relatively rapid succession. In this case theoperative has before him an image in which there is, for example, atraffic sign as the target object 11. A screen 12 is situated betweenthe image and the eye 9. When this screen 12 is actuated, the image infront of the eye 9 is covered. The screen 12 can be, for example, an LCDscreen which can change relatively quickly between a matt, opaque and abright, transparent appearance. There is also a system of two mirrors 2,3, by means of which an image detail can be projected round the outsideof the screen. The idea is that the image detail should compriseessentially only the target object 11, possibly with the immediateenvironment. The target object 11 projected by the mirrors 2 and 3serves as the fixation point for the eye 9 of the operative. If thescreen 12 is now switched on and off alternately, the complete scenewith the target object 11 in the periphery of the field of vision ispresented alternately to the eye 9. The target object 11 remains in thecenter of the field of vision, i.e. on the fovea, by means of mirrors 2and 3. The angle α is taken here as the measure of visual prominence. Adiaphragm, for example, could be considered as an alternative to thescreen 12.

The advantage of such a device is that there is no distortion of theimage presented through lenses and the like, and that a large apertureangle of the field of vision is obtained.

FIG. 3 shows yet a further variant of the device according to theinvention. In the case of this device 1 the image of a scene reaches theeye 9 by way of two light paths 13, 14. For this purpose, the light path13 defined by a mirror 15, an eyepiece 16, mirrors 17, 18, a firstdoublet lens 19, an adjustable diaphragm 20, a second doublet lens 21, avariable beam-splitting mirror 22 and, finally, an eyepiece 23. Theeyepiece 23 contains an adjustable eye fixation feature 24. By way ofthe light path 14, the light reaches the eye 9 after passing insuccession a fixed lens 25, a fixed diaphragm 26, a lens 27 which ismovable in the lengthwise direction of the light path 14, the variablebeam-splitting mirror 22 and the eyepiece 23. The elements situatedalong the light path 13 ensure that the scene, possibly enlarged orreduced, is projected in focus on the retina of the eye 9. On the otherhand, the elements situated along the light path 14 ensure that thescene, possibly enlarged or reduced to the same extent as in the case ofthe light path 13, is projected in focus on the pupil of the eye 9,instead of on the retina. A so-called Maxwellian view is consequentlyachieved with the light path 14, which means that the retina of the eye9 perceives the scene as a sort of mist.

In order to ensure that very conspicuous objects (for example, objectswhich are very close up) do not still remain easily visible when thegreatest possible peripheral shift takes place--with the result thatsuch objects cannot be measured--it is possible to add an imagedegradation combined with the peripheral shift, in such a way that theamount of light in the eye is not affected. For this purpose, contrastreduction of the entire image sector can be used, by means of a roundsemi-transparent mirror which reduces contrast in a linear mannerrunning upwards from 0 to 100%, and which is placed in one of the twolight paths in such a way (see FIG. 3, number 22) that--in combinationwith a lens right next to the eye (see FIG. 3, number 25)--a Maxwellianview occurs in that light path. The mirror can then be turned by handinto the desired contrast reduction position. Another setting method ismechanical connection of the contrast reduction setting device (i.e. themirror) to the peripheral shift. Other contrast reduction methods arealso possible, for example using a linear 0 to 100% transparent frostedglass, instead of the mirror. If such a frosted glass is connectedmechanically to the peripheral shift, one light path alone will suffice,and a Maxwellian view is no longer necessary.

The two light paths 13, 14 can now be combined by means of the variablebeam-splitting mirror 22. The position of that beam-splitting mirror 22determines the ratio between the in-focus and the out-of-focus image onthe eye 9, and thus the contrast of the target object within the scene.The exit pupil of the device 1 is adjustable for both light paths 13,14. For the light being transmitted by way of the light path 13, this isachieved by means of an adjustable diaphragm 20. For the light beingtransmitted by way of the light path 14, this is achieved by means ofthe movable lens 27. The adjustment mechanism of the diaphragm 20 andthe lens 27 respectively are connected, so that the pupil size is alwaysthe same for the two light paths 13, 14. This ensures that the exitpupil of the device is always smaller than the pupil of the eye 9.Depending on the light conditions, the pupil of the eye will be largeror smaller, so that the exit pupil of the device 1 must be adapted toit. The eye fixation feature 24 is a small black dot, to which theobserver can direct the eye 9 during a measurement. Said eye fixationfeature 24 is adjustable by means of a micrometer (not visible). For ameasurement the procedure is as follows: the exit pupil of the device 1is first adjusted depending on the light conditions. The device is thendirected at the scene in such a way that the target object is centeredrelative to the eye fixation feature 24 on which the operative iskeeping his eye 9 directed. The desired contrast can now be set usingthe variable beam-splitting mirror 22. If desired, the contrast settingmay be carried out already before the apparatus 1 is directed at thescene. The operative can then direct his eye 9 at another part of thescene, while continuing to look through the apparatus 1, so that thetarget object is shifted towards the periphery of the field of vision ofthe eye 9. Such a shift can be achieved by either turning the device 1in its entirety through an angle, or by adjusting the eye fixationfeature 24. In both cases the extent to which the target object is movedto the periphery is measurable. The operative can now shift the targetobject towards the periphery of the field of vision of his eye 9precisely so far that the target object is no longer perceptible withthe eye 9, and this gives a measure of the visual prominence. Anotherpossibility is to move the target object towards the periphery of thefield of vision of the eye 9, and then to change the contrast of thetarget object within the scene, by adjusting the beam-splitting mirror22, for example until the target object is no longer perceptible for theoperative. The extent to which the target object is shifted towards theperiphery of the field of vision of the eye 9 in combination with thecontrast at which the target object is no longer visible for theoperative can then be used as a measure of the visual prominence of thetarget object within the scene.

It will be clear to the person skilled in the art that the variant of anembodiment of the device 1 shown in FIG. 3 is essentially the same asthat of FIG. 1 in the situation where the light path 14 is fullyswitched off. In this variant of an embodiment shown in FIG. 1 thetarget object can also be shifted towards the periphery of the field ofvision of the eye 9 by turning the device 1 in its entirety, or byadjusting an eye fixation feature in, for example, the lens 6, insteadof tilting the mirror 3.

The ideas for measuring the visual prominence by adjusting the contrastof the target object within a scene and the idea of presenting to theeye in succession within a short period of time a centered target object(projection onto the fovea) and shifting a scene with the target objecttowards the periphery can also be combined by, for example, combinationof the embodiments shown in FIGS. 1 and 3. For example, the system oflenses, mirrors and diaphragms shown in FIG. 1 is set up in the device 1of FIG. 3 in the part through which the light path 28 travels (thecombination of the light paths 13 and 14 after the variablebeam-splitting mirror 22).

A further variant with the working principle according to the device 1of FIG. 3 could be as follows: the variable beam-splitting mirror 22 isreplaced by a fixed beam-splitting mirror. Sets of a fixed and anadjustable polaroid filter are placed in both the light path 13 and thelight path 14. The adjustable polaroid filters are connected, so thatthey are adjusted to the same extent. The sets of polaroid filters inthe respective light paths 13, 14 are designed in such a way that onsetting of the adjustable polaroid filters the light transmission isincreased along one light path and reduced along the other light path,or vice versa, so that the contrast of the target object within thescene can be changed by changing the ratio between image on the pupil ofthe eye and on the retina of the eye 9. The sets of polaroid filters areformed from, for example, a fixed polaroid filter which is rotatedthrough ninety degrees relative to the fixed polaroid filter in theother light path. The adjustable polaroid filters are also formed by acommon polaroid filter which rotates about its center point and coversboth the light path 13 and the light path 14.

We claim:
 1. A method for determining the visual prominence of a targetobject in an image comprising the steps of:presenting by a display unitan image sector of the image with the target object to an eye of anoperative; placing the fovea of the eye into a reference positionrelative to the display unit; shifting the image sector towards theperiphery relative to the fovea of the eye until the target becomes nolonger visible; and measuring the extent of the shift from the referenceposition to the position where the target becomes no longer visible as ameasure of the visual prominence of the target object.
 2. Methodaccording to claim 1, in which a second image sector of the same imageis formed, in the case of which second image sector the target object ispresented to the fovea, and the two image sectors are presented insuccession in a relatively short time.
 3. Method according to claim 2,in which the second image sector is configured in such a way so as topresent essentially only the target object.
 4. The method of claim 1,comprising the further step of using a display unit that is suitable forpresenting different images to the eye.
 5. The method of claim 1,comprising the further steps of using a display unit with a variablecontrast display, and adjusting the contrast of the variable contrastdisplay so that the contrast of the images is varied with respect to theimage surroundings.
 6. The method of claim 1, wherein the steps ofpresenting by a display unit an image sector of the image with thetarget object to an eye of an operative and placing the fovea of the eyeinto a reference position relative to the display unit, places thereference position within the image sector.
 7. Visual measuring devicefor determining the visibility of a target object in an image, providedwith a display unit for presenting an image sector of an image with atarget object to an eye of an operative, having a reference point fordirecting the fovea of the eye into a reference position relative to theimage sector, having adjusting means for changing the position of thearea of the image to be presented with the image sector relative to thereference point, and having a measuring device for determining theextent to which the target object lies outside the reference point as ameasure for the visibility of the target object, with the display unitequipped in such a way that two image sectors of the same image can bepresented in succession in a relatively short time, with the targetobject in a mutually shifted position.
 8. Device according to claim 7,in which the display unit comprises an optical system with a mirrorelement, which is moveable between two annular positions, for thepurpose of influencing the light transmission with the optical system insuch a way that two image sectors of the same image can be presentedrelatively shortly after one another, with the target object in amutually shifted position, and connected to said element for influencingthe light path an image sector defining element which can assume twodifferent positions, in order to present two image sectors of differentsizes.
 9. The device of claim 8, wherein said image sector definingelement comprises a diaphragm.
 10. Device according to claim 7, in whichthe display unit comprises a display viewing surface with a controllablecontrast designed to adjust the viewed contrast within the image sector.11. Device according to claim 10, in which a light path dividing elementis situated in or on the display unit, in order to split the light pathinto two partial light paths for presentation of the image sector, witha separate optical system for each partial light path, for presentingone partial light path in focus and the other partial light path out offocus, and with a light path combining element downstream of thoseseparate optical systems, in order to combine the two partial lightpaths in an adjustable ratio and present them to the eye.
 12. Visualmeasuring device for determining the visibility of a target object in animage, comprising a display unit for presenting a first image sector ofan image with a target object to an eye of an operative, a referencepoint for directing the fovea of the eye into a reference positionrelative to the first image sector, adjusting means for changing theposition of the area of the image to be present with a second imagesector relative to the reference point, a measuring device fordetermining the extent to which the target object lies outside thereference point as a measure for the visibility of the target object,and a screen element which can assume a light-transmitting displayposition and a non-light-transmitting covering position, in which in thecovering position the first image sector is covered and the second imagesector is presented to the eye via a mirror, around the screen element.